mid-IR Power Research Articles

High-power, high-repetition-rate tunable longwave mid-IR sources based on DFG in the OPA regime.

We demonstrate high-power longwave mid-IR ultrafast sources based on a high-power Er-fiber laser system at 1.55 µm with a 32-MHz repetition rate. Compared with previous 1.03-µm-driven difference frequency generation (DFG), our current configuration allows tighter focusing in the GaSe crystal thanks to an increased damage threshold at 1.55 µm. Consequently, the 1.55-µm-driven DFG can operate in the regime of optical parametric amplification (OPA), in which the mid-IR power grows exponentially with respect to the square root of the pumping power. We experimentally demonstrate this operation regime and achieve broadband mid-IR pulses that are tunable in the 7.7-17.3 µm range with a maximum average power of 58.3 mW, which is also confirmed by our numerical simulation.

Long seed, short pump: converting Yb-doped laser radiation to multi-µJ few-cycle pulses tunable through 2.5-15 µm.

We present a setup for generating broadband (up to 1050 cm-1) and broadly tunable (2.5-15 µm) mid-infrared pulses using an Yb-doped femtosecond laser as the pump source. Our scheme, comprising two parametric amplifiers and a mixing stage, exploits favorable group velocity matching conditions in GaSe pumped at 2 µm to directly produce sub-70 fs pulses throughout the tuning range without any additional dispersion compensation, while 30-50 fs pulse durations are achieved with simple dispersion compensation by propagation through thin bulk media. The generated pulses have sub-1% short- and long-term energy noise, as well as stable spectral parameters, while delivering 0.5-2 W average mid-IR power. We expect the source to be useful for various spectroscopic applications in the mid-IR.

Mid-infrared laser source for testing jamming code effectiveness in the field

We report a mid-infrared (mid-IR or 3.5 to 5.0 μm) laser source for testing jamming code effectiveness against heat-seeking missiles in open-field conditions. We developed an optical parametric oscillator (OPO) based on a ZnGeP2 (ZGP) crystal for converting a 2.1-μm pump laser into mid-IR. The pump laser is an acousto-optically Q -switched Ho:YAG laser pumped by a continuous-wave (cw) Tm:fiber laser. The maximum possible average mid-IR power obtained is 6.5 W at a pump power of 20 W with a 33% power conversion efficiency. The pump pulses resulting from the Q -switched operation of the Ho:YAG laser have a pulse width of 30 ns (FWHM) at a pulse repetition frequency (PRF) of 50 kHz. We characterized the output beam in terms of its output power and wavelength spectrum. The output power of the pump laser can be modulated in a pattern of on/off pulses with a smaller frequency than the PRF. The frequency and the duty cycle of these on/off pulses can be adjusted. Identical on/off pulse modulation appears on the mid-IR output beam after the OPO. It is possible to create jamming patterns and codes in mid-IR band using this feature. We have presented examples of on/off modulation of the OPO output at arbitrary pattern frequencies and duty cycles.

Investigating the structure of the windy torus in quasars

Thermal mid-infrared emission of quasars requires an obscuring structure that can be modeled as a magneto-hydrodynamic wind in which radiation pressure on dust shapes the outflow. We have taken the dusty wind models presented by Keating and collaborators that generated quasar mid-infrared spectral energy distributions (SEDs), and explored their properties (such as geometry, opening angle, and ionic column densities) as a function of Eddington ratio and X-ray weakness. In addition, we present new models with a range of magnetic field strengths and column densities of the dust-free shielding gas interior to the dusty wind. We find this family of models -- with input parameters tuned to accurately match the observed mid-IR power in quasar SEDs -- provides reasonable values of the Type 1 fraction of quasars and the column densities of warm absorber gas, though it does not explain a purely luminosity-dependent covering fraction for either. Furthermore, we provide predictions of the cumulative distribution of E(B-V) values of quasars from extinction by the wind and the shape of the wind as imaged in the mid-infrared. Within the framework of this model, we predict that the strength of the near-infrared bump from hot dust emission will be correlated primarily with L/L_Edd rather than luminosity alone, with scatter induced by the distribution of magnetic field strengths. The empirical successes and shortcomings of these models warrant further investigations into the composition and behaviour of dust and the nature of magnetic fields in the vicinity of actively accreting supermassive black holes.

RADIO–FAR-INFRARED CORRELATION IN “BLUE CLOUD” GALAXIES WITH 0 <z< 1.2

We study the radio--far infrared (FIR) correlation in "blue cloud" galaxies chosen from the PRism MUltiobject Survey (PRIMUS) up to redshift ($z$) of 1.2 in the XMM-LSS field. We use rest-frame emission at 1.4 GHz in the radio and both monochromatic (at 70$\mu$m) and bolometric (between $8-1000~\mu$m) emission in the FIR. To probe the nature of the correlation up to $z\sim1.2$, where direct detection of blue star-forming galaxies is impossible with current technology, we employ the technique of image stacking at 0.325 and 1.4 GHz in the radio and in six infrared bands, viz. 24, 70, 160, 250, 350 and $500~\mu$m. For comparison, we also study the correlation for more luminous galaxies that are directly detected. The stacking analysis allows us to probe the radio--FIR correlation for galaxies that are up to 2 orders of magnitude fainter than the ones detected directly. The $k-$correction in the infrared wavebands is obtained by fitting the observed spectral energy distribution (SED) with a composite mid-IR power law and a single temperature greybody model. We find that the radio luminosity at 1.4 GHz ($L_{\rm 1.4GHz}$) is strongly correlated with monochromatic FIR luminosity at 70 $\mu$m ($L_{\rm 70\mu m}$) having slope $1.09\pm0.05$ and with bolometric luminosity ($L_{\rm TIR}$) having slope $1.11\pm0.04$. The quantity $q_{\rm TIR} (=\log_{10}[L_{\rm TIR}/(3.75\times 10^{12} L_{\rm 1.4 GHz})])$ is observed to decrease with redshift as $q_{\rm TIR} \propto (1+z)^{-0.16\pm0.03}$ probably caused due to the non-linear slope of the radio--FIR correlation. Within the uncertainties of our measurement and the limitations of our flux-limited and color-selected sample, we do not find any evolution of the radio--FIR correlation with redshift.

High peak-power mid-infrared ZnGeP_2 optical parametric oscillator pumped by a Tm:fiber master oscillator power amplifier system

We report on the utilization of a novel Tm:fiber laser source for mid-IR ZnGeP2 (ZGP) optical parametric oscillator (OPO) pumping. The pump laser is built in a master oscillator power-amplifier configuration delivering up to 3.36 W of polarized, diffraction limited output power with 7 ns pulse duration and 4 kHz repetition rate. This corresponds to a peak power of ∼121 kW and a pulse energy of ∼0.84 mJ. With this source, we generated 27.9 kW of total mid-IR peak power in a doubly resonant oscillator (DRO) configuration. This is, to the best of our knowledge, the highest ever demonstrated mid-IR peak power from a directly Tm:fiber laser pumped ZGP OPO. Moreover, a DRO output with about 284 μJ of total mid-IR pulse energy was demonstrated using 100 ns pump pulses. The wavelength tuning of the idler was extended to 6 μm with lower output power in another OPO experiment.

Wildly Tunable, High-Efficiency MgO:PPLN Mid-IR Optical Parametric Oscillator Pumped by a Yb-Fiber Laser

We demonstrate a wildly tunable, high-efficiency mid-infrared (mid-IR) output-coupled single resonant optical parametric oscillator (OC-SRO) pumped by a Yb-fiber laser. The compact mid-infrared source employs a 50-mm-long, multi-grating MgO-doped periodically poled lithium niobate (MgO-PPLN) crystal, providing as much as 1.73 W idler power at 3.012 μm, and 1.27 W signal power at 1645 nm, corresponding to an overall conversion efficiency of 41.7% and a slope efficiency of 77.9%. In particular, the mid-infrared output power of 1.03 W and 0.67 W are obtained at 3.7 μm and 3.9 μm, respectively, with an optical-to-optical conversion efficiency of 14.3% and 9.3%. Furthermore, the idler light is tunable from 3 μm to 3.9 μm by changing the periods from 31 to 28.5 μm, with the output power >1 W over 78% of the tuning range. Our experimental results are pump power limited and further mid-IR power and conversion efficiency could be obtained with a suitable high-power pump source. The total OC-SRO output power rms at 2.6 W is about 0.6% during 2 h measurement.

Air-clad silicon pedestal structures for broadband mid-infrared microphotonics

Toward mid-infrared (mid-IR) silicon microphotonic circuits, we demonstrate broadband on-chip silicon structures, such as: (i) straight and bent waveguides and (ii) beam splitters, utilizing an air-clad pedestal configuration which eliminates the need for typical mid-IR-lossy oxide cladding. We illustrate a sophisticated fabrication process that can create high-quality pedestal structures in crystalline silicon, while preserving its mid-IR transparency. A fundamental waveguide mode is observed between λ=2.5 μm and λ=3.7 μm, and an optical loss of 2.7 dB/cm is obtained at λ=3.7 μm. Our pedestal silicon structures show 50:50 mid-IR power splitting enabling the further development of mid-IR silicon microphotonics.

GOODS-HERSCHEL: SEPARATING HIGH-REDSHIFT ACTIVE GALACTIC NUCLEI AND STAR-FORMING GALAXIES USING INFRARED COLOR DIAGNOSTICS

We have compiled a large sample of 151 high-redshift (z = 0.5–4) galaxies selected at 24 μm (S24 > 100 μJy) in the GOODS-N and ECDFS fields for which we have deep Spitzer IRS spectroscopy, allowing us to decompose the mid-infrared spectrum into contributions from star formation and activity in the galactic nuclei. In addition, we have a wealth of photometric data from Spitzer IRAC/MIPS and Herschel PACS/SPIRE. We explore how effective different infrared color combinations are at separating our mid-IR spectroscopically determined active galactic nuclei from our star-forming galaxies. We look in depth at existing IRAC color diagnostics, and we explore new color–color diagnostics combining mid-IR, far-IR, and near-IR photometry, since these combinations provide the most detail about the shape of a source's IR spectrum. An added benefit of using a color that combines far-IR and mid-IR photometry is that it is indicative of the power source driving the IR luminosity. For our data set, the optimal color selections are S250/S24 versus S8/S3.6 and S100/S24 versus S8/S3.6; both diagnostics have ∼10% contamination rate in the regions occupied primarily by star-forming galaxies and active galactic nuclei, respectively. Based on the low contamination rate, these two new IR color–color diagnostics are ideal for estimating both the mid-IR power source of a galaxy when spectroscopy is unavailable and the dominant power source contributing to the IR luminosity. In the absence of far-IR data, we present color diagnostics using the Wide-field Infrared Survey Explorer mid-IR bands which can efficiently select out high-z (z ∼ 2) star-forming galaxies.

GOODS-HERSCHEL: IMPACT OF ACTIVE GALACTIC NUCLEI AND STAR FORMATION ACTIVITY ON INFRARED SPECTRAL ENERGY DISTRIBUTIONS AT HIGH REDSHIFT

We explore the effects of active galactic nuclei (AGN) and star formation activity on the infrared (0.3-1000 microns) spectral energy distributions of luminous infrared galaxies from z = 0.5 to 4.0. We have compiled a large sample of 151 galaxies selected at 24 microns (S24 > 100 uJy) in the GOODS-N and ECDFS fields for which we have deep Spitzer IRS spectroscopy, allowing us to decompose the mid-IR spectrum into contributions from star formation and AGN activity. A significant portion (~25%) of our sample is dominated by an AGN in the mid-IR. Based on the mid-IR classification, we divide our full sample into four sub-samples: z~1 star-forming (SF) sources; z~2 SF sources; AGN with clear 9.7 micron silicate absorption; and AGN with featureless mid-IR spectra. From our large spectroscopic sample and wealth of multi-wavelength data, including deep Herschel imaging at 100, 160, 250, 350, and 500 microns, we use 95 galaxies with complete spectral coverage to create a composite spectral energy distribution (SED) for each sub-sample. We then fit a two-temperature component modified blackbody to the SEDs. We find that the IR SEDs have similar cold dust temperatures, regardless of the mid-IR power source, but display a marked difference in the warmer dust temperatures. We calculate the average effective temperature of the dust in each sub-sample and find a significant (~20 K) difference between the SF and AGN systems. We compare our composite SEDs to local templates and find that local templates do not accurately reproduce the mid-IR features and dust temperatures of our high redshift systems. High redshift IR luminous galaxies contain significantly more cool dust than their local counterparts. We find that a full suite of photometry spanning the IR peak is necessary to accurately account for the dominant dust temperature components in high redshift IR luminous galaxies.

AEGIS: Radio and Mid‐Infrared Selection of Obscured AGN Candidates

The application of multiwavelength selection techniques is crucial for discovering a complete and unbiased set of active galactic nuclei (AGNs). Here, we select a sample of 72 AGN candidates in the extended Groth strip (EGS) using deep radio and mid-infrared (mid-IR) data from the Very Large Array (VLA) and the Spitzer Space Telescope, and analyze their properties across other wavelengths. Only 30% of these sources are detected in deep 200 ks Chandra X-Ray Observatory pointings. The X-ray-detected sources demonstrate moderate obscuration with column densities of NH 1022 cm−2. A stacked image of sources undetected by Chandra shows low levels of X-ray activity, suggesting they may be faint or obscured AGNs. Less than 40% of our sample are selected as AGNs with optical broad lines, mid-IR power laws, or X-ray detections. Thus, if our candidates are indeed AGNs, then the radio/mid-IR selection criteria we use provide a powerful tool for identifying sources missed by other surveys.

Black hole growth and stellar assembly at high-z

Context. Observations indicate a strong link between star formation and black hole (BH) growth, but some questions remain unanswered: whether both activities are coeval or whether one precedes the other, what their characteristic timescales are, and what kinds of physical processes are responsible for this interplay. Aims. We examine stellar and BH masses (M * and M BH ) in z ∼ 2 active systems at the peak of their AGN or star formation activity to investigate how they are linked and whether AGN radiative or else radio power provides a feedback mechanism that regulates the stellar growth in these systems. Methods. We analyze the infrared (IR) spectral energy distributions of radio, sub-millimeter and mid-IR selected AGNs at z ∼ 1-3 and constrain their stellar and AGN luminosities using AGN and host-galaxy templates. Results. We find evidence of increasing stellar light, thereby decreasing the AGN mid-IR power going from mid-IR selected AGNs, to radio galaxies, and to sub-millimeter AGNs. This trend can be explained by either decreasing Eddington ratios or increasing offsets from the local M BH -M * relation. All systems are characterized by high star formation rates regardless of their different AGN powers, thus neither AGN radiative power nor AGN-driven radio activity seems to influence the star formation rate in the selected AGNs. We discuss two possible evolutionary scenarios that might link these three AGN classes.

High-power, tunable difference frequency generation source for absorption spectroscopy based on a ridge waveguide periodically poled lithium niobate crystal

A novel waveguide for difference frequency generation in the mid-IR spectral region at 3.52 mum is characterized. High mid-IR power of 15 mW and an external conversion efficiency of up to 19 %W( -1) have been obtained. An optical beam propagation factor M(2) =1.18 was determined using the second moment method. A simple 2-f absorption spectra demonstrates the potential of this mid-IR source for high precision trace gas sensing applications.

Watt-level, high-repetition-rate, mid-infrared pulses generated by wavelength conversion of an eye-safe fiber source

We report on a mid-infrared (mid-IR) source consisting of an approximately 10 W average-power, linearly polarized 1.54 microm wavelength pulsed fiber source pumping an optical parametric oscillator. From this source, we obtained average power in excess of 1 W in the 3.8-4.0 microm wavelength range at a pulse repetition frequency of 100 kHz. With a slightly different setup, we also achieved an average power of 0.25 W at 4.5 microm wavelength. To our knowledge, these values represent the highest mid-IR power obtained through wavelength conversion of an eye-safe fiber source.

Performance Characteristics of Narrow Linewidth Fiber Laser Pumped Mid-IR Difference Frequency Mixing Light Source for Methane Detection

A high-power, narrow-linewidth Yb fiber laser with a fiber Bragg grating (FBG) pumped difference frequency generation (DFG) in a periodically poled lithium niobate (PPLN) crystal was investigated in detail. A mid-IR power of ∼2.3 µW at 3.3 µm with a slope efficiency of 0.85 mW/W2 was achieved. A Doppler-broadened absorption spectrum of CH4 at 3038.497 cm-1 (3.2911 µm) was obtained with a 0.1-m long-gas cell at a pressure of 133 Pa. The linewidth of the DFG source was evaluated to be less than 96 MHz from the observed spectral linewidth. Real-time monitoring of CH4 (∼1.78 ppm) in ambient air in a multipass cell which has an optical path length of 10 m was also demonstrated.

Trace-gas sensor based on mid-IR difference-frequency generation in PPLN with saturated output power

Abstract The development and characterization of a compact pulsed mid-IR laser source for sensitive on-line trace-gas analyses in the 3–4 μm wavelength range is reported. The source is based on an advantageous difference-frequency mixing configuration in periodically poled LiNbO 3 (PPLN) with a cw external-cavity diode laser (ECDL; 810–830 nm) for broad and accurate tunability and a diode-pumped passively Q-switched Nd:YAG laser (1064 nm) for high mid-IR peak power. With 5 mW cw pump and 4.7 kW signal peak power incident on a 19 mm long PPLN crystal, a maximum of 360 μW mid-IR peak power was generated. The narrowband (∼150 MHz) radiation was saturated by a factor of 15 compared with the nonsaturated case due to depletion of the pump laser radiation. This results in a very high amplitude stability of the generated mid-IR power and thus in a high detection sensitivity. A minimum detectable absorption coefficient of 2.8×10 −8 cm −1 was achieved in combination with a 36.2 m multipass cell in an averaging time of 20 s, as demonstrated by on-line analyses of formaldehyde traces near 3.53 μm.

Broadly tunable mid-IR radiation source based ondifferencefrequency mixing of high power wavelength-tunable laserdiodes in bulk periodically poled LiNbO 3

Coherent mid-IR radiation throughout the 3.6 to 4.3 µm wavelength range is generated by difference frequency mixing (DFM) of wavelength-tunable laser diodes in periodically-poled LiNbO3 (PPLN). Mid-IR power levels up to 7.1 µW and DFM conversion efficiencies up to 0.015%/W cm are demonstrated.